Phase-resolved thermal modeling of a fluorescent lamp electrode as a function of current waveshape and frequency

A modeling technique for calculating approximate phase-resolved temperature profiles of a fluorescent lamp electrode is presented. Analytical expressions for instantaneous electrode temperature are derived by solving a set of simplified differential equations describing time-domain electrode temperature dynamics. The electrode surface temperature fluctuations are a function of current waveform and operating frequency. The model shows that moderate temperature fluctuations indicate safe operating conditions as far as lamp life expectancy is concerned. It is found that at low frequency operation, distorted current waveforms, i.e. higher crest factor values, generate larger electrode surface temperature swings within a single alternating-current period. Comparison of the temperature fluctuations resulting from low and high-frequency modulated operation indicates a need to impose restrictions on the modulation depth when the high-frequency modulated current technique is used in ballast implementation. This is due to the fact that the overall surface-temperature-fluctuation profile of the electrode is determined by the low-frequency components of the lamp driving waveform. By judicious choice of modulation depth, a modulated high-frequency ballast that will result in guaranteed rated lamp life equivalent to that obtained with traditional low-frequency ballasts can be designed, even with a modulated lamp current.<>